Bug 413369

SUMMARY

I'm getting a warning "WARNING: unhandled amd64-darwin syscall: unix:151"
whenever I'm calling getpgid under valgrind.

STEPS TO REPRODUCE
```
#include <stdio.h>
#include <unistd.h>

int             main()
{
        pid_t pgid;

        if ((pgid = getpgid(0)) == -1)
                perror("getpgid");
        else
                printf("pgid : %d\n", pgid);
}
```

OBSERVED RESULT
I'm getting the pgid properly whenever I'm just running the program alone.
With valgrind, perror is printing me `getpgid: Function not implemented`.

EXPECTED RESULT

I should always have my pgid.

SOFTWARE/OS VERSIONS
Windows: 
macOS: Sierra 10.12.6
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION
Personnal stackoverflow question.
https://stackoverflow.com/questions/58525060/getpgid-not-implemented-with-valgrind/58526688#58526688